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Question
A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.5 W cm−2. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
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Solution
Given:-
Radius of the sphere, r = 1 cm
Intensity of light, I = 0.5 Wcm−2
Let A be the effective area of the sphere perpendicular to the light beam.
So, force exerted by the light beam on the sphere is given by,
`F = P/c = (AI)/c`
`F = (pi xx (1)^2 xx 0.5)/(3 xx 10^8)`
`= (3.14 xx 0.5)/(3 xx 10^8)`
`= 0.523 xx 10^-8`
`= 5.2 xx 10^-9 "N"`
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